ORGANIC LETTERS
Isolation and Characterization of Unsymmetrical C60Me5O3H, a Cage-Opened Bisepoxide Ketone: Tautomerism Involving a Fullerene Cage Bond
2001 Vol. 3, No. 11 1669-1671
Hamad Al-Matar, Ala’a K. Abdul-Sada, Anthony G. Avent, and Roger Taylor* The Chemistry Laboratory, CPES School, UniVersity of Sussex, Brighton BN1 9QJ, U.K.
[email protected] Received March 21, 2001
ABSTRACT
Bisepoxide ketone C60Me5O3H, possessing a nine-membered hole in the cage, has been isolated from the reaction of C60Cl6 with methyllithium followed by hydrolysis. It is a tautomer of the recently isolated bisepoxide fullerenol, this tautomerism being the first example involving a cage C−C bond, and may be driven by cage strain. Like the fullerenol, the ketone gives a high C58+ fragmentation ion intensity during EI mass spectrometry.
The formation of cage-opened fullerene ketones was first conjectured by Chiang et al. to account for the carbonyl stretching frequencies observed with some hydroxylated fullerenes (fullerenols) on treatment with acid.1 The observed carbonyl stretching frequencies of the ketones (believed to be derived from hemiketals moieties in the fullerenols) were at unusually high wavenumbers (1722 and 1726 cm-1). This is attributable to the strain in the fullerene cage, an explanation invoked by Wudl et al. to account for the CdO frequency (1727 cm-1) of a characterized cage-opened ketone formed by oxidation of an N-methoxyethoxymethylazahomo[60]fullerene.2 Recently we described the isolation and full (1) Chiang, L. Y.; Upasani, R. B.; Swirczewski, J. W.; Soled, S. J. Am. Chem. Soc. 1993, 115, 5453. See also: Chiang, L. Y. in The Chemistry of Fullerenes; Taylor, R., Ed.; World Scientific: Singapore, 1995; Chapter 5. (2) Hummelen, J. C.; Prato, M.; Wudl, F. J. Am. Chem. Soc. 1995, 117, 7003. 10.1021/ol0100531 CCC: $20.00 Published on Web 04/26/2001
© 2001 American Chemical Society
characterization of Cs-symmetric C60Me5O2OH (2) the first bisepoxy fullerenol.3 From the same reaction (C60Cl6/MeLi followed by hydrolysis) we have now isolated an unsymmetrical isomer (1), a cage-opened ketone formed evidently via a 1,3-tautomeric shift of the hydroxyl hydrogen in the fullerenol. The reaction was carried out as described previously,3 and after column chromatography (70-230 mesh silica gel) with toluene (byproducts having been removed with toluene/ cyclohexane) the toluene eluent was purified by high-pressure liquid chromatography (HPLC). Use of a 10 mm × 250 mm Cosmosil “Buckyprep” column operated at a flow rate of 4.7 mL min-1, with elution either by toluene or toluene/ heptane, (1:1 v/v) yielded the ketone 1 in low yield. This (3) Al-Matar, H.; Hitchcock, P. B.; Avent, A. G.; Taylor, R. Chem. Commun. 2000, 1071.
Figure 1. Structures for the ketone C60Me5O3H (1) and fullerenol C60Me5O2OH (2) showing NOE couplings (%).
eluted after 5.2 min (toluene) or 10.1 min (1:1 toluene/ heptane) [cf. 4.8 min (toluene)/8.8 min (1:1 toluene/heptane) for the fullerenol 2]. The EI mass spectrum (70 eV, Figure 2) shows the parent ion at 844 amu, and notably, the C58+ fragmentation ion at 696 amu shows an intensity (55%) relative to that of the 720 amu peak, cf. 40% noted for the fullerenol 2.3 It arises from the facile loss of 2 CO molecules, which we have noted previously in the mass spectra of phenylated epoxides of [60]fullerene, where the relative intensity was 30%.4 The loss of both CO and CO2 by thermal fragmentation of fullerene oxides can be monitored by a matrix-isolation technique.5 It is apparent from the intense 28 and 44 amu peaks in the low mass region of the mass spectrum (inset to Figure 2). (On heating to 250 °C for 3 h, a KBr disk of 1 also showed the formation of the matrix-isolated CO2 band at 2336 cm-1.)
Figure 2. EI mass spectrum (70 eV) for C60Me5O3H (1); inset shows low mass region. 1670
Figure 3. 1H NMR spectrum for C60Me5O3H (1).
The 1H NMR spectrum (Figure 3, CDCl3) shows peaks at δ 5.60 (1 H, s, cage-H) and 2.37, 2.34, 2.24, 2.08 and 2.04 (each 3 H, s), confirming that the compound is unsymmetrical. [Owing to a lock signal malfunction, the 1H NMR reported3 resonances for 2 should each be downfield by 0.37 ppm, i.e., at δ 4.25 (OH), 2.36 (MeA), 2.23 (MeC), and 2.12 (MeB).] The OH peak in 2 and other fullerenols we have isolated (δ 3.59 and 4.49), all exhibit saturation transfer to water, a reliable test for the presence of OH groups; the peak at δ 5.60 in 1 (which is significantly further downfield) does not. The IR spectrum (Figure 4) shows an intense band at 1703 cm-1, whereas by contrast the spectrum for 2 shows none, having instead an intense broad band at 3520 cm-1. Structure of 1. Salient features in deducing the structure are (i) it is unsymmetrical; (ii) the IR spectrum shows a very strong CdO band; (iii) the NMR singlet at δ 5.6 is characteristic of hydrogen attached to the cage, e.g., literature values are 5.2 (C60Ph5H, CS2)6 and 4.5 (C60Me5H, CDCl3);7 (iv) the singlet shows no saturation transfer to water and is not therefore part of an OH group; (v) the NOE couplings (Figure 1) show that the hydrogen giving the singlet must be located (a) within the ring of (4) Darwish, A. D.; Birkett, P. R.; Langley, G. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Fullerene Sci. Technol. 1997, 5, 705. (5) Taylor, R.; Penicaud, A.; Tower, N. J. Chem. Phys. Lett. 1998, 295, 481. (6) Avent, A. G.; Birkett, P. R.; Crane, J. D.; Darwish, A. D.; Langley, G. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. J. Chem. Soc., Chem. Commun. 1994, 1463. (7) Sawamura, M.; Toganoh, M.; Kuminobu, Y.; Kato, S.; Nakamura, E. Chem. Lett. 2000, 270. Org. Lett., Vol. 3, No. 11, 2001
Figure 5. Motif present in the N-methoxyethoxy-ketolactam derivative of [60]fullerene.
Figure 4. IR spectrum (KBr) for C60Me5O3H (1).
methyl groups, (b) on the central pentagon, (c) off center, (d) nearer to MeB than to MeA. The data are self-consistent since attachment of H to the cage requires (given the molecular formula) a carbonyl group to be present as observed. Moreover, the chemical shifts of the B/B′ methyls (average 2.06 ppm) in 1 are similar to those corresponding in 2 (2.12 ppm), the shifts for the C/C′ methyls (average 2.29 ppm) are similar to those corresponding in 2 (2.23 ppm), and the shift of the MeA is the same in each. This indicates similar locations of the methyl groups in each compound. The very weak B/B′ and C/C′ NOE couplings also parallel the insignificant B/C coupling in the (X-ray characterized) 2.
Org. Lett., Vol. 3, No. 11, 2001
The structure for 1 is the only one either feasible or possible. Furthermore, the carbonyl group is in a sixmembered ring and thus is subject to less strain than if it were in a five-membered ring, as in the ketones of Chiang et al.1 and Wudl and co-workers,2 and hence appears at a lower wavenumber than in these examples. [Compare also the IR carbonyl frequencies for cyclopentanone (1746 cm-1) and cyclohexanone (1714 cm-1).8] Isomers 1 and 2 are therefore tautomers and this tautomerism appears to be the first example involving breakage of a fullerene cage C-C bond. It may be driven by reduction of strain in the five-membered central ring of the fullerenol; the bond that breaks in the fullerenol is longer (1.569 Å) than the others in the five-membered ring (1.484 and 1.532 Å).3 Finally, the “facing location” of the CdO and C-H groups in 1 has a parallel in the similar locations of CdO and NR (more sterically crowded) in the motif (Figure 5) for the N-methoxyethoxy-ketolactam precursor that is the precursor for the formation of aza[60]fullerene.2 Acknowledgment. We thank the University of Kuwait for a research grant (to H.A.-M.). OL0100531 (8) Aldrich Library of FT IR Spectra; Pouchet, C. J., Ed.; Aldrich: Milwaukee, 1985.
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